Summer 2001

Forest-to-cropland shift affects Midwestern temperatures

by Stephen Cole
NASA Goddard Space Flight Center

(Photo by Carlye Calvin.)

The large-scale conversion of forests to croplands in the
midwestern United States over the last century has led to a
measurable cooling of the region's climate, according to NCAR
scientist Gordon Bonan. The study, which appeared in the June
issue of the Journal of Climate, is the first to document
the link between regional climate change and a major change in
temperate forest cover.

"Human uses of land, especially clearing of forest for agriculture
and reforestation of abandoned farmland, are an important cause of
regional climate change," concludes Bonan. The cooling is the
result of the changeover of the region to crops, which reflect
more sunlight back into space than forests.

The impact of land-use changes on climate is currently one of the
most uncertain factors contributing to climate warming, according
to the recent third assessment report from the Intergovernmental
Panel on Climate Change. Most of the work to date on this subject
has been with computer models and has focused on deforestation in
the tropics in areas such as the Amazon. Bonan's is one of the
first observational studies on the effect of temperate-forest
changes on regional climate. His own earlier model results hinted
at this cooling effect in the U.S. Midwest, but, explains Bonan,
"I wanted to see if this pattern really exists in nature."

Since accurate temperature and land-use records do not exist for
the Midwest 150 years ago, when agricultural clearing began to
deforest the region, Bonan relied on a direct modern-day
comparison between temperatures in predominantly forested areas
and those in cropland areas to see if the different types of land
cover were associated with different temperatures.

He used temperatures from 65 U.S. weather reporting stations from
1986 to 1995, where the surrounding land cover was either crops or
forests and there were no nearby cities or water bodies, which can
have their own distinct effects on temperatures. The cropland
sites were predominantly in the Midwest, where 80% of the land is
now under cultivation; the forested stations were in the
Northeast, where just 20% of the land is now agricultural.

The data showed that the daily temperature range was lower in the
Midwest than in the forested Northeast. This was because the
daytime heating of agricultural stations across the Midwest was
consistently lower than that of the forested northeastern
stations. The result was a surprise, because previous regional
climate studies showed that the Midwest should have a larger daily
temperature range than the Northeast, due to the moderating
influence of clouds on daytime heating. The eastern United States
is generally cloudier than the Midwest, and more clouds reflect
more solar energy back into space. Bonan's study found that
temperatures in the Midwest did not rise as much during the day as
they did in the Northeast, contrary to what was expected from
these regional differences in cloud cover.

The seasonal character of the differences in temperatures between
the agricultural Midwest and forested Northeast suggested a strong
influence by the land cover. Bonan found that the cooling was most
prominent in the Midwest in late spring and summer, just when
crops reached their full growth. The temperature difference
diminished in the fall, after harvest time.

In 1850 croplands were on their way to being the dominant land
cover in the Northeast, but forests and grasslands still dominated
the Midwest, with only 5% of the land under cultivation. Just 30
years later, when northeastern croplands reached their highest
level, the Midwest had caught up to match the Northeast, with both
at 50% of land under cultivation. As cropland then steadily
declined in the Northeast, forests returned; the spread of
midwestern agriculture continued for the next 100 years, peaking
at about 80% in the 1980s.

To make sure that the results he was seeing were not happening in
only one decade of the temperature and land-use measurements,
Bonan also analyzed a 100-year record of U.S. temperatures. Before
1940, when the two regions had more similar amounts of cropland,
the difference in regional daily highs was much smaller than it is
today. Since 1940, as agriculture continued to spread across the
Midwest and northeastern farm lands returned to forests, the
temperature difference steadily increased. The Northeast became
warmer in the spring and summer as forests returned.

Bonan is currently using a computerized model of the climate to
further investigate the impact of historical deforestation on the
eastern United States. This project is funded by NASA's Earth
Observing System.